Motor vehicles having a hybrid drive are discussed in the German patent document DE 10241018 A1, for example. Referred to as hybrid drives are drives in which at least one electro machine is provided in addition to a conventional internal combustion engine; depending on the design of a parallel hybrid drive, this electro machine can be coupled or is fixedly coupled to a drive shaft in the drive train of the motor vehicle and can operate both as generator or as motor. In generator operation, the electro machine is driven by the internal combustion engine and is thus able to generate electric current to supply electric loads of the motor vehicle. In motor operation, current is supplied from the vehicle battery in order to convert this current into driving energy for the motor vehicle or into starter energy for the internal combustion engine. Apart from the vehicle battery as energy store, a flywheel or some other store for kinetic energy may be provided as well, by which kinetic energy released during braking, for example, is storable and can be released via the electro machine to the loads in the vehicle electrical system of the motor vehicle or later to its drive train.
An electric driving operation requires a closed-loop speed control of the electro machine, especially in a number of cases. For an automatic transmission having an open mechanical lockup clutch, and for the oil-pressure supply of a conventional automatic transmission, an input speed of >0 is required, and in electrical driving operation, even for a stationary vehicle, a rotational speed of the electro machine of >0 is therefore required. With an automatic transmission and an open mechanical lockup clutch, the drive-away behavior of a conventional motor vehicle in a torque-controlled operation of the electro machine is representable only inadequately. The driver is expecting a crawl torque when disengaging the brake. In the case of an open mechanical lockup clutch, this requires an electro machine speed of >0, even if the vehicle is at a standstill. To ensure a behavior that corresponds to the standing-start behavior of the combustion engine out of idle speed control of the internal combustion engine, the electro machine must be operated in an rpm-controlled manner. In the case of a hybrid vehicle having manual gear-shifting, the drive-away behavior during electric operation should not differ from the behavior with a running internal combustion engine. The driver is used to a drive-away behavior against an rpm-controlled power unit, using the starting clutch. A drive-away in a torque-controlled operation of the electro machine, starting from a rotational speed of 0, i.e., with a starting clutch that is not depressed and with the gear engaged, is unfamiliar to the driver and should be avoided.
The hybrid operation likewise requires a closed-loop speed control of the electro machine. For future diagnostic and monitoring functions, the electro machine drags the internal combustion engine to a specifiable rotational speed, for example in a drag torque adaptation in no-load running, the internal combustion engine being operated using deceleration fuel cutoff. If corresponding functions are implemented during no-load running, then the electro machine must take over the idle speed control of the internal combustion engine. Depending on the operating state of the vehicle, it will be necessary to operate the internal combustion engine and the electro machine jointly or separately in an rpm-regulated manner; if a torque is requested, for example by the actuation of the driving pedal, a steady transition into the torque-controlled operation must take place.